1. Field
The present invention relates generally to telecommunication systems, and more specifically to scheduling techniques in such systems.
2. Background
Enhancements to third-generation (3G) data services for code-division multiple access 2000 (cdma2000) and wideband code-division multiple access (WCDMA) are being developed. Such enhancements are called 1x evolution for data and voice (1xEV-DV) and high-speed downlink packet access (HSDPA), respectively. In these systems, the telecommunication network has almost total control in terms of dynamically assigning the available spreading code space and transmission energy to mobiles in a time scale of one to two milliseconds. A mobile may continuously provide feedback to the network in the form of an indicator to help the network optimize the use of resources. To effectively operate such networks, it is advantageous for a base station to serve only a small number of users at any given time, especially to those users who indicate that their channel conditions are good. The data rate of transmissions can be chosen such that the frame error probability is reasonable.
In the above-mentioned services, each user is assigned a unique spreading code when they have information to send. The same frequencies can be used by each user, because the receiver, which is listening for that user's specific code, is able to effectively ignore other users' signals as long as the codes are orthogonal. A problem with this solution is that multipath propagation is prevalent in the wireless environment, and causes the code sequences to deviate from being orthogonal. This results in interference between users within a cell called “multiple access interference”, which ultimately limits the capacity of the cell. Consequently, if the radio propagation between a base station and a remote station exhibits multipath, then the energy spent on common channels and other users will interfere with the desired code channel.
There is therefore a need in the art for scheduling to transmit data packets in such a way as to reduce the multiple access interference.